This invention relates to guided wave radar measurement instruments and, more particularly, to a probe for a guided wave radar measurement instrument.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and lost cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe, that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
The design of a GWR liquid level measurement probe for steam and other high temperature/pressure applications requires a seal assembly that satisfies both mechanical and electrical requirements. Mechanically, the seal must simultaneously be resistant to the corrosive effect of steam, withstand process temperatures of 600xc2x0 F. and above, pressures in excess of 2000 psi, and carry the tensile and bending loads induced by a center conductor extending into the process vessel. Electrically, the seal assembly must be electrically transparent to the radar signal to allow proper functioning of the system. In addition, the mechanical design affects the electrical characteristics so that the two are interrelated.
Plastic materials, such as PTFE, are often used as process seals for lower temperature and lower pressure probes. Plastic has a low dielectric constant. This permits the design to achieve the needed electrical impedance within dimensional constraints imposed by conventional xc2xexe2x80x3 inch NPT process connections while maintaining adequate structural integrity. Sealing of the plastic process seal is usually performed by o-rings. However, plastic material and o-rings are not capable of withstanding the high temperatures encountered in steam service, and the like.
Ceramic materials offer high temperature capability and resistance to temperature and steam. However, sealing ceramics to metals is difficult and the ceramic materials are susceptible to cracking under thermal shock. Typically, ceramic seals are brazed to the metal. However, the coefficients of thermal expansion for ceramics are much less than for high temperature and pressure steels. The large differences in the amount that the ceramic and steel expand and contract between the braising, steam service and room temperature results in large stresses. These stresses tend to damage the ceramic and/or the braised joint. Even a pin hole leak in the ceramic or braising will cause the probe to fail. Compensating for the thermal expansion differences results in complex and costly seal designs.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.
In accordance with the invention, a probe is reliably sealed without susceptibility to thermal shock while providing transparency for electrical signals.
Broadly, in accordance with one aspect of the invention, a probe defines a transmission line for use with a measurement instrument including a pulse circuit connected to the probe for generating pulses on the transmission line and receiving reflected pulses on the transmission line. The probe includes a center conductor for conducting the pulses. A conductive outer sleeve is coaxial with the center conductor. The conductive outer sleeve has a process end and a connector end. The process end is exposed, in use, to a process environment being measured. A first cylindrical seal element between the center conductor and the outer sleeve is at the sleeve process end. The first cylindrical seal element is of a first material adapted to withstand a relatively high temperature. A second cylindrical seal element is between the center conductor and the outer sleeve disposed between the first cylindrical seal element and the connector end. The second cylindrical seal element is of a second material adapted to withstand a lower temperature than the first cylindrical seal element.